Move HugoSites to hugolib

It will get more involved in the build process in a minute.

See #2309

hugolib/hugo_sites.go

@@ -0,0 +1,25 @@
+// Copyright 2016-present The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package hugolib
+
+// HugoSites represents the sites to build. Each site represents a language.
+type HugoSites []*Site
+
+// Reset resets the sites, making it ready for a full rebuild.
+// TODO(bep) multilingo
+func (h HugoSites) Reset() {
+	for i, s := range h {
+		h[i] = s.Reset()
+	}
+}